Many devices for use in optical communication networks utilize electrically controllable, birefringent elements, generally liquid crystal (LC) elements for performing signal modulation functions, generally by means of changing the phase between the ordinary and the extraordinary components of the optical beam transmitted through the element, and hence the polarization direction of the beam. These applications are generally achieved using a single liquid crystal element for each optical function to be accomplished. In the prior art there are described numerous examples of such applications, including for instance, those described in PCT Application. No. PCT/IL/02/00511 for Wavelength Selective Optical Switch, and in PCT Application No. PCT/IL/02/00188 for Fiber Optical Attenuator, published as WIPO document WO 02071133, and in PCT Application No. PCT/IL/02/00187 for Dynamic Gain Equalizer, published as WIPO document WO 02071660, and in PCT Application No. PCT/IL/02/00167 for Fiber Optical Gain Equalizer, published as WIPO document WO 03009054, all of which are incorporated herein by reference, each in its entirety.
Systems which use multiple liquid crystal elements have also been described in the prior art, such as that described for the purpose of pulse shaping in the article entitled “Programmable phase and amplitude femtosecond pulse shaping” by M. W. Wefers and K. A. Nelson, published in Optics Letters, Vol. 18, No. 23, pp. 2032-2034, 1993. In this application, although two LC elements are used serially as spatial light modulators in a pulse shaping application, each would appear to fulfil a different system function. Thus, one is used as a phase mask, modulating the relative phases of the different dispersed frequency components of the beam, while the other acts as an amplitude mask, which attenuates the different frequency components of the beam.
In prior art applications and systems using liquid crystal (LC) elements, each optical signal modulation or processing function is generally fulfilled by a single LC element, and each system module may include a number of such functions. However, this generally results in limitations on the desired functionality because of intrinsic limitations of each liquid crystal element. Such limitations can arise from a number of possible sources. Thus, for instance, there may be limitations in the overall phase shift which can be generated in the LC element, because of the nature of the behavior of birefringence as a function of applied voltage. Additionally, there may be limitations to the temperature stability of the device because of the temperature coefficient of the birefringence in the liquid crystal material. When temperature stability is important, prior art applications often utilize temperature stabilization of the entire circuit module using internal heaters, which complicates and increases the cost of the circuit module. Furthermore, most liquid crystal materials have wavelength-dependent operation, since such materials are generally dispersive. The efficiency of the circuit function in which they are used is thus wavelength dependent, and in, for instance, a prior art liquid crystal based channel blocking module, the attenuation at the center of the waveband may be higher than that at the band edges since the phase change or polarization rotation generated in the LC material cannot generally be optimized for all wavelengths. Adjustment of the switching voltage in such devices for each separate wavelength-dispersed pixel is not generally a simple or cost-effective solution.
Furthermore, since each different type of LC material—whether nematic, twisted nematic, smectic, chiral nematic, or any other type—has its own functional limitations, and the way in which the material is operable in the LC element may also be functionally limited, the available range of suitable LC materials may not enable attainment of the exact optical functionality desired. Furthermore, the alignment of the optical axis of the LC element, as defined by the rubbing direction, also generally limits the available use configuration of the element. Additionally, there may be spatial limitations to the pixel patterns useable on prior art single LC devices, and this may limit their applicability for some system requirements.
There therefore exists a need for liquid crystal devices which can operate without the limitations of the kinds mentioned hereinabove, or at least with reduced limitations, in order to enable the construction of more specifically suited devices for use in dedicated optical processing systems.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.